Radio-frequency power amplifiers are normally used both in mobile terminals and in base stations for mobile radios. Particularly in the case of mobile radio units, it is desirable to keep the power consumption of the entire arrangement as low as possible in order to increase the life of the rechargeable battery, and thus the time for which the radio can be used for speech and on standby. The power consumption of a radio transmitter is governed mainly by the power consumption of the power amplifier.
Modern rechargeable batteries which are suitable for use in mobile radios are designed, for example, using lithium-ion technology, or the like. Batteries such as these have the disadvantage that they have a relatively wide voltage range, which extends from 4.5 V when the rechargeable battery is fully charged, down to 3V when the rechargeable battery is discharged. Future generations of lithium batteries with an even greater storage capacity per unit volume are predicted to have an even wider voltage range from 2 V to 5 V.
Owing to the trend towards mobile radios which cope with ever more applications at the same time, an increasing bandwidth and an ever greater data rate, their power consumption is also increasing.
The relatively wide voltage range in which the electrical charge store emits its energy leads, however, to the radio-frequency power amplifier in particular, the so-called RF-PA, which is normally connected directly to the rechargeable battery, having a widely fluctuating supply voltage. The fluctuating supply voltage leads inter alia to problems with power matching. The output impedance of the amplifier must be matched to the impedance of the antenna. Even now, the output impedance fluctuates from about 2.53 Ω at 4.5 V to 1.13 Ω at 3 V, depending on the supply voltage. If the voltage range with which the power amplifier is supplied becomes even wider in the future, then the output impedance will fluctuate from about 3.13 Ω at 5 V to 0.5 Ω at 2 V. This results in a factor of 6.25 between the maximum and minimum output impedance that occurs. However, this will have an adverse effect on the efficiency of the power amplifier over the entire rechargeable battery voltage range.
The reason why the radio-frequency power amplifier is normally connected directly to the rechargeable battery is to avoid any additional voltage drop resulting from series regulators and to avoid the efficiency losses caused by voltage converters. The article by: M. Jordan: Single Inductor, Tiny Buck-Boost Converter Provides 95% Efficiency in Lithium-Ion to 3.3 V Applications—Design Note 275, Linear Technology Corporation, California, USA 2002 describes a power supply for a WCDMA (Wideband Code Division Multiple Access) mobile radio and its power amplifier.
The power matching of the power amplifier to a transmission antenna is normally optimized at the required transmission arrangement rated power. The efficiency at the rated power is accordingly comparatively high, but decreases relatively sharply towards lower transmission power levels.